High temperature metallic diffusion coating

ABSTRACT

Improved oxidation and sulfidation resistance can be imparted to a high temperature metallic diffusion coating diffusion bonded with metallic surface, such as nickel, cobalt or iron base alloy surface, by embedding in a coating outer portion between about 5 and 50 volume percent of oxide particles of up to about 20 microns in size and thermally stable to at least about 2,000*F.

United States Patent [1 1 Levine HIGH TEMPERATURE METALLIC DIFFUSION COATING [75 Inventor: David J. Levine, Cincinnati, Ohio [73] Assignee: General Electric Company, Cincinnati, Ohio [22] Filed: Sept. 28, 1970 21 App1. No.: 76,278

Related U.S. Application Data [62] Division of Ser. No. 780,177, Nov. 29,1968, Pat. No.

521 U.S. c|.; ..29/195 [51] Int. Cl. ..B32b 15/00 [58] Field of Search ..29/l95; 148/315 [5 6] References Cited UNITED STATES PATENTS 2,775,531 12/1956 Montgomery et a1. ..29/195 X 1 March 6, 1973 2,900,276 8/1959 Long et a1 ..29/l95 X 3,415,672 12/1968 Levinstein et al. ....29/198 X 3,477,831 11/1969 Talboom et a1. ..29/195 Primary Examiner-L, Dewayne Rutledge Assistant Examiner-E. L. Weise Attorney-Derek P. Lawrence, E, S. Lee, 111, Lee H. Sachs, Frank L. Neuhauser and Oscar B. Waddell 5 7 ABSTRACT volume percent of oxide particles of up to about 20 microns in size and thermally stable to at least abou 2,000F.

3 Claims, No Drawings HIGH TEMPERATURE METALLIC DIFFUSION COATING This is a divisional application of application Ser. No. 780,177 filed Nov. 29, 1968 now US. Pat. No. 3,617,360 dated Nov. 2, 1971 and assigned to the assignee of the present invention.

One manner for increasing the operating life of a high temperature operating component in modern power producing apparatus such as turbine engines is to interdiffuse with the components surface a protective high temperature coating. A number of these types of coatings have been reported and are well known in the coating art as high temperature diffusion type coatings.

A specific type of coating to which the present invention relates is described by co-pending application Ser. No. 693,691 filed Dec. 14, 1967 now [7.5. Pat. No. 3,540,878 issued Nov. 17, 1970 and assigned to the assignee of the present invention. Described in that copending application is a ternary alloy useful in a multicomponent particulate mixture and in a metallic diffusion type coating method. The alloy consists essentially of, by weight, about 50 70% Ti, 48% Al and 0.5 9% combined carbon and has a dispersion of Ti AlC complex carbide in a matrix of Ti or Al or their alloys. The particulate mixture includes, along with the alloy and an inert filler, about 0.1 10 weight percent of a halide salt activator which will react with a metallic element in the ternary alloy to form a halide of such metallic element under sufficient time and temperature conditions in a non-oxidizing atmosphere. Preferred are chlorides and fluorides of ammonium and of the alkali metals of Group I A of the Periodic Table of Elements. Specifically preferred is about 0.1 2 percent of halides selected from NaF, KF, NH Cl and NI-l F.

The diffusion type coating of unusual characteristics which results from use of that described method is presently being applied in production use. However, despite the benefits of the coating defined by the above identified co-pending application and other similar coatings, it has been recognized that still further improvement is desired, not only to resist oxidation but also to resist sulfidation, sometimes referred to as hot corrosion.

It is a principal object of the present invention to improve the coating of the type which results from practice of the invention of the above identified co-pending application by providing a coating of an improved combination of oxidation and sulfidation resistance.

Another object is to provide such an improvedcoating having an outer portion which will act as an interdiffusion barrier between oxidizing and sulfidizing environments around a coated article and the article basis metal.

These and other objects as well as the advantages of the present invention will be more fully understood from the following detailed description and examples which are typical of the present invention defined by the appended claims.

It has been recognized that an improved combination of oxidation and sulfidation resistance can be obtained in a diffusion bonded coating of about 1 5 mils as measured in the as-coated condition by embedding in an outer portion of the coating, such as the one described in the above identified co-pending application, about 5 50 volume percent of entrapped oxide Particles of about 1 20 microns in size and therm ing of about 1 25 mils in thickness of such oxide particles in a non-fused, loosely held, vapor permeable condition and of a type which will not enter into the reaction of the metallic diffusion coating. Then the metallic diffusion coating, such as defined by the above identified co-pending application, penetrates through the particles of the interim coating entrapping the particles in the coating outer portion during the heating cycle. Because the oxide particles are about I 20 microns in size and thus are larger than a size which would allow them to move interstitially into the basis metal or alloy, the oxide particles are confined to the coating outer portion.

The useful life of metallic diffusion coatings applied to high temperature operating components such as of a gas turbine engine is dependent in a significant amount on the stability of the interface between the coating and the basis metal to which it has been applied. Of course, such coating life is further dependent upon coating surface degradation resulting from contact with erosive, corrosive or oxidizing environments. However, inhibition of diffusion through the coating of oxidizing and sulfidizing agents toward the coating basis metal interface can significantly lengthen the useful life of the coating. The present invention recognizes that the introduction of the above defined oxide particles in an outer portion of the coating results in such a barrier to that undesirable migration.

As a specific example, one form of the above described ternary alloy powder used in the evaluation of the present invention consisted nominally, by weight, of 61% Ti, 34% Al, and about 4.5% combined carbon. This ternary alloy in powder form will be hereinafter identified as Alloy C. Prior to the preparation of specimens for the evaluation of the present invention, a blend of about 40 weight percent Alloy C powder and about 60 weight percent A1 0 powder was mixed with about 0.2 weight percent NH,F. In this type of particulate mixture, the A1 0 powder acts to inhibit sintering of the Alloy C powders but does not enter into the coating process.

The method of the present invention contemplates the application to the surface of an article to be diffusion coated of oxide particles, stable both to the processing temperature and mechanism, as an interim or preliminary coating in a non-fused, loosely held vapor permeable condition. This can be and was in this example accomplished by first making a slurry of the oxide particles and a liquid binder which will decompose without residue upon heating. For example, an acrylic resin in toluene or acetone can be used. As will be discussed later, the thickness of the interim coating is not critical. Therefore, the slurry can be adjusted in concentration to provide, for example, from about 1 25 mils of oxide on the surface. However, from a practical viewpoint no more than about 15 mils is necessary. Although the oxide particlesare no more than about 20 microns in size, it is preferred that a variety in size of particles be present rather than particles all of the same size.

A variety of oxides of thermal and process stability, including oxides of titanium, aluminum and their mixtures, have been evaluated in connection with the present invention. It is preferred that TiO, or A1 0, or their mixtures in particle sizes of about 1 20 microns be used in the present invention when coating surfaces based on one of the elements Fe, Ni or C0.

The slurry can be applied to the article surface by a variety of methods such as brushing, dipping, spraying, etc. In this example, a paint spray gun was used with a slurry to apply a coating of a powder mixture of about 50 weight percent each of A1 0, and TiO After application of the oxide coating to the article surface to be coated, the article was placed in a container, such as retort, along with the previously described particulate mixture, including powdered Alloy C. It should be recognized, however, that as indicated before, the A1 in the particulate mix is included as a filler and does not enter into the coating reaction. Therefore, ifa method such as is described in co-pending application Ser. No. 780,199, now US. Pat. No. 3,598,638 issued Aug. 10, 1971, filed concurrently with this application, in which the article surface to be coated does not physically contact the coating material such as the particulate mixture, then the filler may be eliminated if particle sintering is not a problem. If the article is to be contacted by the particulate mixture as described in the above identified co pending application Ser. No. 693,691 then inclusion of the A1 0 filler is preferred.

The retort enclosing the particulate mixture and the article surface including the interim oxide coating was provided with a non-oxidizing atmosphere, in this example hydrogen, and was heated in the range of 1,400 2,100F for l 4 hours, for example, about 1,950F for 3 4 hours. This time and temperature was sufficient to allow deposition of such elements as aluminum andtitanium from powdered ternary Alloy C, through their halide vapor state, onto the article surface by first penetrating through and subsequently entrapping the particles of the oxide of the interim coating. The time and temperature selected must be sufficient to allow further interdiffusion between the deposited elements and the article surface.

After cooling, the treated article surface was removed from the retort. Loosely adhering excess powders either from the particulate mix, if the article contacted such mix, or from the interim oxide coating, or both, were removed from the coated surface. It has been found that although relatively thick interim coatings of oxides can be applied to the surface, the titanium or aluminum or both from the ternary alloy powder will penetrate through the excess oxides to leave them only loosely adherent and readily removable from the surface after processing. Thus a reasonaplied as the loosely adherent, non-fused, vapor permeable particles comprising the interim coating.

Practice of the method of the present invention provides an article surface, particularly one based on Fe, Ni or Co, with a diffusion bonded, two portion coating: a coating outer portion and a coating inner interdiffusion portion. Each portion includes primarily intermetallics formed from the basis element, for example, Ni, Co or Fe, and certain of any alloying elements of the article surface, with metallic diffusion coating elements such as titanium, aluminum, etc. The intermetallics result from interdiffusion between elements of the coating and of the basis metal or alloy. As was stated before, the entrapped oxide particles in the coating outer portion, because of their size in the range of about 1 20 microns, cannot move interstitially with respect to the basis metal in the formation of coating inner interdiffusion portion between the outer portion of the coating and the article surface. Therefore only the coating outer portion includes, in addition to the interrnetallic, entrapped oxide particles.

It has been found in the evaluation of the present invention that the coating inner interdiffusion portion and the coating outer portion together in the as-coated condition must represent a thickness no greater than about 5 mils. Thicknesses in excess of that amount result in coatings having poor adherence properties.

it has been recognized further that amounts of oxide particles less than about 5 volume percent provide no benefit over a coating without such oxide particles. Amounts of entrapped oxide in excess of about 50 volume percent results in significantly reduced coating erosion resistance. Therefore, the present invention contemplates a coating the outer portion of which includes, in addition to the above described intermetallics and diffusion products, about 5 50 volume percent of dispersed oxides.

The oxides preferred for dispersion in the outer portion of the coating in the practice of the method of the present invention in coating Fe, Ni or C0 base alloys, include Al,O or TiO, or their mixtures. If such a mixture of oxides is used, it has been found that about 50 percent by weight of each is a satisfactory mixture and that a varied size mixture of particles is preferred in the range of l 10 microns.

As was shown in the above identified co-pending application Ser. No. 693,691, the type of diffusion coating with which this invention is particularly concerned can be applied to alloy surfaces based on elements selected from Ni, Co and Fe. Typical of data generated in the evaluation of the present invention is that relating to high temperature nickel base superalloys, the compositions for which are shown in the following Table I as including such alloying elements as Cr, Co,

Mo, W, Al, Ti, Fe, etc:

TABLE I Composition \vt. percent-Dal. Ni and incidental impurities Cr 13 Co Mo Ti Al Zr V Cb/Ta W Fe Max. ble excess thickness of the interim coating is not critical. It is preferred that about 2 20 mils of oxide be ap- Specimens of each of the nickel base alloys listed in Table I were coated in the manner described in the above specific example: A mixture of 50 percent by volume each of Al,0,, and of TiO, of particle size in the range of l microns was applied as an interim coating by spraying from a slurry of the oxide particles and acrylic resin in acetone. The coating method was conducted at about 1,950F for 3 4 hours in a hydrogen atmosphere. The resulting coating had a coating outer portion, including the entrapped oxides, of about 1 mil thick and a coating inner interdiffusion portion of about 1 mil thick for a total coating thickness of about 2 mils. The oxide particles in the coating outer portion TABLE II STATIC OXIDATION: 2,100F 100 hrs.

% Coating Consumed As shown from the data of Table ll, both coatings protected base metal adequately for 100 hours at 2,l00F. However, the life of the coating including the entrapped oxide particles in the coating outer portion is significantly greater as evidenced by less than 1 percent of it being consumed compared with consumption of 50 75 percent for the non-oxide bearing metallic diffusion coating. Therefore, the improved coating of the present invention would continue to protect the article many times longer than the same coating without embedded oxides.

Another series of specimens coated in the same manner were placed in a hot corrosion testing apparatus cycled between about 1,650 and l,725F. Heat was generated from combustion of a natural gas/air mixture. At the same time, 100 parts per million of an aqueous sulfidizing corrodant containing about 9% NaCl and about 10% Na,SO was injected toward the specimens. Results of the hot corrosion testing is shown in the following Table III.

TABLE III 100 l-lr. Cyclic l-lot Corrosion Test Coating Life (hours) Basis Alloy Metal alone Metal Oxide l 25 I00 2 I5 I00 3 I0 100 4 45 I00 no failure at end of I00 hour test.

As can be seen by the data of Table lll, in the hot sulfidizing atmosphere described, the life of the coating of the present invention is significantly better than the same coating without entrapped oxides, even though both coatings were capable of withstanding more than hours under oxidizing conditions alone as shown by the data from which Table II was selected.

. Thus, although the coating described by the above identified co-pending application Ser. No. 693,691 has significant benefits and uses particularly under oxidizing conditions, it has been recognized that the practice of the present invention provides an unexpected and significant improved combination of hot corrosion or sulfidation resistance along with oxidation resistance.

What is claimed is:

1. An article having a surface including a diffusion bonded coating of about 1 5 mils as-coated of im proved resistance to oxidation and sulfidation, comprising:

a metallic base portion including a basis metal element' a coatirig outer portion consisting essentially of intermetallics of the basis element (a) with at least one element selected from the group consisting of Al and Ti and (b) with elements selected from the group consisting of the alloying elements of the base portion, the coating outer portion including about 5 50 volume percent of embedded oxide particles of about 1 20 microns in size and thermally stable at least to about 2.,000F; and

a coating inner interdiffusion portion diffusion bonded with both the base portion and the coating outer portion and comprising intermetallics of the basis element (a) with at least one element selected from the group consisting of Al and Ti and (b) with elements selected from thegroup consisting of the alloying elements of the base portion.

2. The article of claim 1 in which:

the metallic base portion is an alloy based on a basis element selected from the group consisting of Fe, Ni and Co; and

the embedded oxide particles are at least one oxide selected from the group consisting of oxides of Al and Ti and are included in the coating outer portion in the amount of 10 40 volume percent.

3. The article of claim 2 in which:

the alloy of the base portion is based on the element nickel; and

the oxide particles are about 1 10 microns in size. 

1. An article having a surface including a diffusion bonded coating of about 1 - 5 mils as-coated of improved resistance to oxidation and sulfidation, comprising: a metallic base portion including a basis metal element; a coating outer portion consisting essentially of intermetallics of the basis element (a) with at least one element selected from the group consisting of Al and Ti and (b) with elements selected from the group consisting of the alloying elements of the base portion, the coating outer portion including about 5 -50 volume percent of embedded oxide particles of about 1 - 20 microns in size and thermally stable at least to about 2,000*F; and a coating inner interdiffusion portion diffusion bonded with both the base portion and the coating outer portion and comprising intermetallics of the basis element (a) with at least one element selected from the group consisting of Al and Ti and (b) with elements selected from the group consisting of the alloying elements of the base portion.
 2. The article of claim 1 in which: the metallic base portion is an alloy based on a basis element selected from the group consisting of Fe, Ni and Co; and the embedded oxide particles are at least one oxide selected from the group consisting of oxides of Al and Ti and are included in the coating outer portion in the amount of 10 - 40 volume percent. 